The present invention relates to a digital signal processing apparatus in an image transmission system, such as digital CATV or digital satellite broadcasting, in which a time-base compressed television signal is transmitted. The present invention reduces the time between the user transmitting a program request and viewing the requested program.
Previously, only analog audio/video signals were transmitted from a broadcasting station to the public. But in recent years, broadcasting stations have begun to transmit digitized signals in association with the development of digital techniques. These digital techniques compress a video signal transmitted from a broadcasting station so that signals which were conventionally transmitted in approximately ten analog channels are now transmitted in one analog channel. The compressed digital signal can be further time-base compressed before it is transmitted. For example, when a video signal which was compressed at a rate of 1.5 Mbps is transmitted on a transmission path which can transmit actual digital data, excluding an error correcting code and the like, at 24 Mbps, the video signal can be further time-base compressed by a factor of sixteen. Therefore, a program of two hours can be transmitted in seven or eight minutes.
FIGS. 7 and 8 illustrate systems in which the time-base compressed signal is transmitted. In FIG. 7, each home transmits a request for a program to program distributing center 101 over the telephone lines. The time-base compressed digital signal is transmitted to communication satellite 103 through parabolic antenna 102 by the program distributing center. The communication satellite transmits the received digital signal to parabolic antenna 104 of cable TV center 105 and/or parabolic antenna 107 of each home 106. Cable TV center 105 relays the received digital signal to the contracting home 108 via a CATV line.
The user cannot view the program before the signal is processed since the received digital signal has been time-base compressed. The digital signal received by each home 106, 108 is recorded by a recording apparatus such as a digital VTR or the like and, after the recording of the program is complete, the digital signal is reproduced and returned to its original time-base for viewing. Therefore, each home must have a receiver connected to a recording and reproducing apparatus.
Each home, or consumer, pays a charge through financial facilities 109 when a program is requested. Thus, accounting information is stored in an accounting data base (not shown) of program distributing center 101 when the received digital signal data are recorded by the recording and reproducing apparatus in that home.
FIG. 8 illustrates a block diagram of program distributing center 101. An audio/video signal from signal source 110a, such as a VTR, an optical disc, or the like, is compressed by MPEG encoder 111a. The compressed signal is divided into packets by packet producing circuit 112a and is time-base compressed by time-base compression circuit 113a. In FIG. 8, a plurality of channels are processed in this manner, as represented by elements 110b, 111b, 112b and 113b, and one channel is selected by multiplexer 114 for further processing and transmission. The multiplexed signal selected for transmission is enciphered by scramble processor 116 with a scramble key generated by conditional accessing circuit 115. The scramble key can be transmitted to each home over the telephone lines or it can be transmitted with the multiplexed signal. An error correction code is added to the enciphered signal by error processing circuit 117 and the signal is modulated for transmission by modulating circuit 118. The modulated signal is transmitted via output terminal 119.
According to the U.S. patent application Ser. No. 08/253,283 filed on Jun. 12, 1994, by the present applicant, the user must wait at least seven or eight minutes from the transmission of a program request until the user is able to view the program, assuming a program length of two hours and a time-base compression factor of sixteen. The delay is caused because the digital signal is not reproduced until after the entire program has been recorded on the recording and reproducing apparatus at his home.